1. Field of the Invention
The present invention pertains to a method of etching a shaped cavity in a substrate. In particular, the present invention pertains to a method for etching a shaped cavity, where the width of the shaped cavity is equal to or greater than the depth of the shaped cavity in the substrate.
2. Brief Description of the Background Art
Commonly owned, copending U.S. application Ser. No. 09/372,477, filed Aug. 11, 1999, of Podlesnik et al., provides a general method for forming a multi-part cavity in a substrate which is useful in many different micromachining applications. As disclosed by Podlesnik et al., a substrate is etched to a first predetermined depth to form a shaped opening. A conformal protective layer is then formed on at least the sidewall of the shaped opening. The protective layer comprises a material which has a different etch selectivity than the substrate material. If necessary, the protective layer is then anisotropically etched to remove portions of the protective layer which overlie the bottom of the shaped opening. Typically, at least a portion of the substrate at the bottom of the shaped opening is exposed prior to proceeding with subsequent etching. The substrate is then further etched to form a shaped cavity using an etchant gas which selectively etches the substrate relative to the protective layer.
For some applications it is desirous that the shaped cavity be formed to have a different shape than the shaped opening. For example, the opening to the cavity is tubular-shaped, while the shaped cavity has a width that is equal to or greater than its depth (i.e., a round or horizontal elliptical shaped cavity). A shaped cavity of such dimensions is particularly difficult to form using dry etching techniques. Formation of round shaped cavities has previously been accomplished using wet etch techniques. However, in recent years, the semiconductor manufacturing industry has been trending toward the use of dry etch techniques because of process integration and environmental considerations. Due to the difficulty of carrying the gaseous reactants into and reaction byproducts out of a cavity, the use of dry etch techniques has been limited to the widening of already formed shaped cavities by a relatively minor amount. Therefore, it would be desirable to provide a dry etch method that would result in the formation of a shaped cavity having a width that is equal to or greater than its depth. Lateral (as opposed to vertical) etch is dependent upon a number of factors during the etching process, including the incoming angle of the etchant species, the mean free path of the etchant species, and the ability of the etchant species to reach the surface to be etched. Furthermore, byproducts of the etch process must constantly be removed, and if these byproducts become trapped within a cavity being etched, etching may slow to an unacceptable rate or stop entirely, particularly in the lateral direction. When a conventional etch process is used, the desired lateral etch (if it can be achieved before etching stops) is typically accompanied by an undesired deeper vertical etch. Therefore, particularly during the etching of buried cavities, it is important to provide a means by which etch byproducts can be evacuated from the etch cavity during the etching process, in order to allow the desired amount of lateral etching without deepening of the cavity.
The present invention provides a method of dry etching a shaped cavity in a substrate. The method is useful for aspect ratios as high as at least 3.5:1, and is particularly useful where the width of the shaped cavity is equal to or greater than the depth of the shaped cavity in the substrate, i.e. where the aspect ratio is less than 1.
We have discovered that it is necessary to control the process chamber pressure in a particular manner during the performance of the etching process to permit etch byproduct removal from the shaped cavity at a rate which reduces or avoids the buildup of etch byproducts on interior surfaces of the shaped cavity. This permits continued etching of the shaped cavity. In general, the method of the invention comprises etching of a shaped cavity using at least two different process chamber pressures, including an initial process chamber pressure, followed by continued etching of the shaped cavity using at least one decreased process chamber pressure. The decreased process chamber pressure should be at least 25% lower than the initial process chamber pressure, and in several embodiments is about 30% to 50% lower than the initial process chamber pressure. An optional finishing or rounding of the etched cavity may be carried out after shaping of the cavity, in which the process pressure is increased; the amount of increase is typically up to about 90% of the initial process chamber pressure.
During the etching of a shaped cavity, the process chamber pressure may be lowered and then subsequently raised and relowered, as well, to provide for the removal of etch byproducts during a given etching step.
The method of the invention is particularly useful in the etching of buried cavities. The shaped cavity is typically formed to underlie a previously formed shaped opening.
The method of the invention can be performed as a continuous process, where the process chamber pressure is gradually lowered or raised and lowered, or as a multi-step process.
When the substrate is (single crystal)silicon, etching is typically performed using a plasma containing reactive fluorine species. The etchant plasma is commonly generated from a source gas comprising SF6 and argon, provided at a flow rate which is process equipment dependent, and at an SF6:argon ratio within the range of about 10:1 to about 2.5:1. When a round shaped cavity is desired, an SF6:argon ratio of about 4:1 provides excellent results. Argon is used as an inert carrier for the SF6, and ionized argon in conjunction with a substrate bias may be used to drive reactive fluorine species generated from SF6 down through the shaped opening and into the shaped cavity.
Alternative primary etchant gases include gases such as CF4, Cl2, and HBr. Any of the primary etchant gases may be used alone or in combination with a compatible other primary etchant gas. For example, CF4 may be used to replace SF6 or may be added to SF6 to obtain a desired effect. A primary etchant gas may be used in combination with an additive gas, such as, for example, Ar, O2, HBr, Cl2, or N2, to provide better control over the surface finish or etch profile of the shaped cavity. Depending on the particular effect desired, the additive gas may be present during the entire duration of the method of the invention, or it may be present only during a certain step or steps. The plasma source gas may further include a substantially nonreactive, diluent gas, such as Ar, He, or Xe.
Examples of source gas combinations which are preferred for etching a silicon substrate, and not by way of limitation, include SF6/Ar/O2; SF6/Ar/HBr; SF6/Ar/Cl2; and SF6/Cl2. CF4 may replace SF6 or may be added to SF6 in any of these source gas combinations.
When the substrate is polysilicon, the plasma source gas typically includes SF6 and/or Cl2. For a silicon dioxide substrate, the plasma source gas typically includes CF4 or NF3, and etching is typically performed at a temperature within the range of about 50xc2x0 C. to about 100xc2x0 C. When the substrate is silicon nitride, the plasma source gas typically includes SF6. When the substrate is a metal (such as aluminum or an aluminum alloy), the plasma source gas typically includes Cl2. When the substrate is polyimide, the plasma source gas typically includes CF4 and O2. All of these examples are intended to be non-limiting.
When a fluorine-containing etchant plasma is used at a temperature below 50xc2x0 C., the protective/masking layer preferably comprises silicon oxide. A silicon nitride protective/masking layer may be used if the plasma source gas does not contain fluorine. A metal or alloy protective/masking layer may be used if the plasma source gas does not contain chlorine.